Damascene processing is commonly used in semiconductor wafer processing to fill vias and trenches formed in a semiconductor substrate with a conductive metal such as tungsten, aluminum or copper for signal routing on the chip. The damascene process typically involves deposition and patterning of a dielectric layer, followed by deposition of barrier and seed layers. Metal is then deposited using chemical vapor deposition (CVD), physical vapor deposition (PVD) or electrochemical deposition (ECD). The deposited metal is then typically planarized by chemical mechanical polishing.
Defects in the metal fill can occur during metal deposition processes used for gap filling. These defects may take the form of voids or weak adhesion at the metal-semiconductor interface, and may degrade chip performance to an unacceptable level. Detection of these defects before further wafer processing takes place eliminates waste and increases efficiency in chip production. Current methods for detecting such defects, such as SEM inspection and electrical testing of the wafer, are time-consuming and not amenable to nondestructive, in-line testing of the full wafer. Other proposed testing methods, such as acoustic detection and positron annihilation spectroscopy, have not demonstrated sufficient sensitivity or throughput to justify industry adoption.